The present invention relates generally to a tubular implantable prosthesis such as vascular grafts and endoprostheses formed of porous polytetrafluoroethylene. More particularly, the present invention relates to a multi-layered tubular self-sealing graft or endoprosthesis formed from primarily expanded polytetrafluoroethylene.
It is well known to use extruded tubes of polytetrafluoroethylene (PTFE) as implantable intraluminal prostheses, particularly vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatability. PTFE tubes may be used as vascular grafts in the replacement or repair of a blood vessel as PTFE exhibits low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tubes. These tubes have a microporous structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft.
Grafts formed of ePTFE have a fibrous state which is defined by interspaced nodes interconnected by elongated fibrils. The spaces between the node surfaces that is spanned by the fibrils is defined as the internodal distance (IND). A graft having a large IND enhances tissue ingrowth and cell endothelization as the graft is inherently more porous.
The art is replete with examples of microporous ePTFE tubes useful as vascular grafts. The porosity of an ePTFE vascular graft can be controlled by controlling the IND of the microporous structure of the tube. An increase in the IND within a given structure results in enhanced tissue ingrowth as well as cell endothelization along the inner surface thereof. However, such increase in the porosity of the tubular structure also results in reducing the overall radial tensile strength of the tube as well as reducing the ability for the graft to retain a suture placed therein during implantation. Also, such microporous tubular structures tend to exhibit low axial tear strength, so that a small tear or nick will tend to propagate along the length of the tube.
The art has seen attempts to increase the radial tensile and axial tear strength of microporous ePTFE tubes. These attempts seek to modify the structure of the extruded PTFE tubing during formation so that the resulting expanded tube has non-longitudinally aligned fibrils, thereby increasing both radial tensile strength as well as axial tear strength. U.S. Pat. No. 4,743,480 shows one attempt to reorient the fibrils of a resultant PTFE tube by modifying the extrusion process of the PTFE tube.
Other attempts to increase the radial tensile, as well as axial tear strength of a microporous ePTFE tube include forming the tubular graft of multiple layers placed over one another. Examples of multi-layered ePTFE tubular structures useful as implantable prostheses are shown in U.S. Pat. Nos. 4,816,338; 4,478,898 and 5,001,276. Other examples of multi-layered structures are shown in Japanese Patent Publication Nos. 6-343,688 and 0-022,792.
Artificial bypass grafts are often used to divert blood flow around damaged regions to restore blood flow. Vascular prostheses may also be used for creating a bypass shunt between an artery and vein. These bypass shunts are often used for multiple needle access, such as is required for hemodialysis treatments. These artificial shunts are preferable to using the body""s veins, mainly because veins may either collapse along a puncture track or become aneurysmal, leaky or clotted, causing significant risk of pulmonary embolization.
While it is known to use ePTFE as a vascular prosthesis, and these vascular prostheses have been used for many years for vascular access during hemodialysis, there remain several problems with these implantable ePTFE vascular access grafts. One major drawback in using ePTFE vascular grafts as access shunts for hemodialysis is that because of ePTFE""s node-fibril structure, it is difficult to elicit natural occlusion of suture holes in the vascular prosthesis made from ePTFE tubing. As a result, blood cannot typically be withdrawn from an ePTFE vascular graft until the graft has become assimilated with fibrotic tissue. This generally takes 2 to 3 weeks after surgery. Furthermore, ePTFE""s propensity for axial tears make it undesirable as a vascular access graft, as punctures, tears, and other attempts to access the blood stream may cause tears which propagate axially with the grain of the node fibril structure.
Providing a suitable vascular access graft has also been attempted in the prior art. Schanzer in U.S. Pat. No. 4,619,641 describes a two-piece coaxial double lumen arteriovenous graft. The Schanzer graft consists of an outer tube positioned over an inner tube, the space between being filled with a self-sealing adhesive. The configuration of this coaxial tube greatly increases the girth of the graft, and limits the flexibility of the lumen which conducts blood flow. Herweck et al., in U.S. Pat. No. 5,192,310 describes a self-sealing vascular graft of unitary construction comprising a primary lumen for blood flow, and a secondary lumen sharing a common sidewall with the primary lumen. A non-biodegradable self-sealing elastomeric material is disposed between the primary and secondary lumen.
While each of the above-referenced patents disclose self-sealing vascular grafts, none disclose a tubular access graft structure exhibiting enhanced radial tensile strength, as well as enhanced resistance to axial tear strength. Furthermore, the multi-layered ePTFE tubular structures and vascular access grafts of the prior art exhibit smaller microporous structure overall, and accordingly a reduction in ability of the graft to promote endothelization along the inner surface. Furthermore, Schanzer does not provide a self-sustaining resealable layer, but rather an elastomeric layer which xe2x80x9cfillsxe2x80x9d the area between the two tubes.
It is therefore desirable to provide a self-sealing ePTFE graft for use in a human body which exhibits increased porosity especially at the inner surface thereof while retaining a high degree of radial strength at the external surface thereof. The graft may preferably be used as a vascular access graft.
It is further desirous to produce an ePTFE vascular access graft which exhibits increased porosity at the outer surface thereof while retaining a high degree of radial tensile and suture retention strengths.
It is still further desirous to provide a self-sealing graft with increased resistance to axially propagating tears.
It is an advantage of the present invention to provide a self-sealing ePTFE graft with increased resistance to axially propagating tears.
It is a further advantage of the present invention to provide a self-sealing ePTFE graft providing superior assimilation capabilities and resealable properties.
It is a further advantage of the present invention to provide a self-sealing ePTFE vascular graft exhibiting an enhanced microporous structure while retaining superior radial strength.
It is a still further advantage of the present invention to provide an ePTFE tubular structure having an inner portion exhibiting enhanced porosity and an outer portion exhibiting enhanced radial tensile strength, suture retention, and suture-hole elongation characteristics.
It is yet another advantage of the present invention to provide a multi-layered ePTFE tubular vascular graft having an inner layer which has a porosity sufficient to promote cell endothelization and an outer layer having a high degree of radial tensile strength.
It is an additional advantage of the present invention to provide a multi-layered ePTFE tubular vascular access graft having an outer layer whose porosity is sufficient to promote enhanced cell growth and tissue incorporation, hence more rapid healing, and an inner layer having a high degree of strength.
In the efficient attainment of these and other advantages, the present invention provides a self-sealing ePTFE graft comprising a first expanded polytetrafluoroethylene (ePTFE) tubular structure having a first porosity, a second ePTFE tubular structure having a second porosity less than said first porosity, said second ePTFE tubular structure being disposed externally about said first ePTFE tubular structure to define a distinct porosity change between said first and second tubular structures, and a resealable polymer layer interposed between said first and second ePTFE tubular structures.
In another embodiment, the present invention provides an ePTFE self-sealing graft, the graft formed of a first ePTFE tubular structure, a second ePTFE tubular structure disposed externally about said first ePTFE tubular structure, and further including a self-sustained resealable polymer layer interposed between the first and second ePTFE tubular structures.
The ePTFE self-sealing graft preferably may be used as a vascular access graft. As more particularly described by way of the preferred embodiment herein, the first and second ePTFE tubular structures are formed of expanded polytetrafluoroethylene (ePTFE). Further, the second ePTFE tubular structure is adheringly supported over the first ePTFE tubular structure to form a composite tubular graft. The strength of this adhesion can be varied as desired to control the characteristics exhibited by the resultant composite structure.
In its method aspect, the present invention provides a method of forming a self-sealing ePTFE graft. The method includes the steps of providing a first ePTFE tubular structure having a desired porosity and strength combination. A second ePTFE tubular structure is provided, also having the desired porosity and strength combination. The second ePTFE structure is disposed over the first ePTFE so as to define a composite vascular graft.
The method of the present invention also provides for the positioning of an intermediate structure between the first and second ePTFE tubular structures. Examples of such structures include an additional ePTFE layer and fibers or thin films of PTFE or other suitable polymers. This intermediate structure also contributes to the resultant porosity and strength of the vascular graft. This intermediate structure can also preferably be a resealable polymer layer interposed between the first and second ePTFE tubular structures.